Yachting's next revolution?

We look at Hugh Welbourn's Dynamic Stability System - a lateral retractible foil to leeward...

The yachting world is on the verge of its next significant revolution with a new invention that adds stability when sailing.

Over the last decades keels have got deeper, their weight concentrated in a lead bulb at their extremity. Water ballast systems have since allowed sea water to be pumped into tanks up to weather, at times adding more than 50% to the displacement of the boat. This allowed bulbs to get smaller and overall displacement to be reduced when unballasted, but has the downside of requiring displacement to increase substantially (as the water ballast is pumped on board) when additional righting moment is required – not the best solution off the wind or in a moderate sea state.

Canting keels, where the bulb is cranked up to weather (first introduced in the Mini class in the early 1980s and adopted by the Open 60s over the course of the 1990s) neatly solved this inherent weakness of water ballast. Since then centreline water ballast tanks have appeared on canting keel Open 60s, used to alter overall displacement and to adjust fore and aft trim. While a canting keel and centreline water ballast configuration ticks most of the boxes in terms of achieving a boat that has a mode for every point of sail, wind strength and sea state, this set-up still has its downsides.

There are several aspects of a canting keel/centreline water ballast set-up that are plain inefficient. For example the canting foil in this set-up should perhaps be more accurately described as a ‘strut’, as all it does is hold the bulb out to weather. Its function of providing lift and preventing leeway, as it would on a fixed keel boat, is vastly diminished when canted and so something must be fitted to compensate – and in the majority of cases this means the addition of two long asymmetric daggerboards fitted either side or just ahead of the mast. The result is that compared to a fixed keel arrangement there is the additional weight of the canting keel mechanism and two extra appendages, plus additional drag of the extra foil/daggerboard that is in the water. While these weight and drag hits may be acceptible evils for most, the desire to remove weight from the strut holding out the bulb has resulted an unacceptible number of failures – Ecover, Skandia and Sill in the last Vendee Globe, movistar in the Volvo Ocean Race, the Skandia maxi in the Sydney Hobart, etc.

It is with these limitations of the canting keel in mind that British yacht designer and foil specialist Hugh Welbourn has come up with the Dynamic Stability System, that achieves everything a canting keel can, outstripping the performance of a canting keel almost all of the time and achieving this in a manner that is significantly more reliable.

As with most good ideas, the DSS is deliciously simple. While water ballast and canting keels both effectively apply downward force to the weather side of a yacht, the DSS uses a foil protruding from the leeward side (like an aircraft wing) to create lift. The faster the DSS boat sails, the more it wants to heel, but this is countered by more lift being generated by the foil to leeward.

Obvious questions

So what does it look like? Potentially it can come in several forms, but the 27ft demonstator/development boat fitted with the DSS foil launched a year ago - that we sailed both on Lake Garda last year (when it was all top secret) and more recently in the south of Spain - is fitted with a single, double ended foil that slides in an athwartships-mounted case through the bilge of a boat. On first impression the movable lateral foil reminded us of the sliding seat on an International Canoe, only that it is a foil not a seat and while the foil is above the waterline when the boat is at rest, it is submerged as the boat heels when sailing. During tacks or gybes the foil is simply pulled through the boat and out on to the opposite side, operated by control lines in the cockpit.

See video of the board retracting here

So the first question - doesn’t it develop significant lee helm or yawing? For some reason associated perhaps with the foil being a hydrodynamically efficent section, it doesn’t. In fact when deploying the foil the feeling is bearly noticable through the helm.

The second question – isn’t it a tad draggy in the light? No, because in the light the foil is simply retracted back into the hull.

The third question – what happens if you wallop the foil on something solid? The worst that can happen is that it snaps off. The foil is designed to break before its case does and if this happens the yacht is still left with its keel intact, only its ‘turbo’ is incapacitated. For this reason the creators of DSS have managed to get the insurance company Pantaneus to endorse the product.



Sold yet? While the system has a significant performance benefit, it has potential advantages for cruising yachts in that it creates a more comfortable ride. The foil reduces heel, but it also dampens roll and a less obvious side affect is that it is also very noticable when on board is how much it dampens pitching. On race boats the benefit of both these features is that they improve the efficency of the rig by creating a more stable flow over the sails.

An additional overall benefit is it reverses the upward weight-power spiral that is very evident at present in the IMOCA Open 60 class. The DSS promotes lightweight easily driven boats that don’t necessarily require massive beam and the weight associated with this. With the IMOCA 60s for example rig heights and sail areas have increased substantially with the latest generation, but imagine a boat that didn’t require a massive aircraft carrier-style hull and a canting keel. Instead, additional stability could be derived from the foil to leeward and as a result the boat could have a narrower hull, more easily driven, thus requiring a smaller sail plan. The end result would certainly be cheaper too because of this. For the 60s, the whole package would be considerably more managable for a solo sailor, who could drive it closer to its full potential. Volvo 70 crews might grudgingly admit this to be an attraction too....

To be honest if there is a downside to this system we haven’t thought of it.

The enigmatic designer

So how did this all come about? Firstly a little about Hugh Welbourn. Based in the West Country, Welbourn is that rare animal, a yacht designer who has raced and still races very regularly. Past accolades from his dinghy days included five back to back wins in the National 12’s Barton Trophy, plus designing, building and sailing in most of the popular development classes, plus stints in the FD and Soling as part of the UK Olympic squad. At the other end of the spectrum, perhaps least known about him is that he sailed around the world doublehanded on a boat called Gazelle, as an unofficial entry in the first Whitbread back in 1973.

Welbourn was particularly active in the heyday of IOR, having sailed in eight Admiral’s Cups, eight Sardinia Cups and seven SORCs, while as a designer it was from this period he gained a reputation as ‘the man to go to’ to have one’s boat optimised for a rating rule. Under IOR he tweaked high profile yachts like Yeoman XXIII, Phoenix, Jamarella and Ragamuffin along with the Bermuda Race winner Condor. For the last IOR Admiral’s Cup he had optimised one third of the fleet.

Post IOR, Welbourn was responsible for the conversions of boats such as Longobarda and the one tonners Fram and Hero from IOR to IMS, along with the IMS optimisation of the winning UK Commodores Cup entries Arbitrator, Wolf, Babalaas and Easy Oars.

Alongside his rule optimisation work, Welbourn has also developed a reputation as a leading appendage specialist and was one of the first to implement CFD in foil analysis (the first boat to benefit from this was Graham Walker’s IOR 50 Indulgence). His list of success as a foil designer is as diverse as it is impressive – including the appendages for Shirley Robertson’s Europe and her subsequent Yngling and Iain Percy’s Star foils for Athens, the keel for Adam Gosling’s 1995 Etchells Worlds winner, to some of Nigel Irens’ 60ft trimarans such as Region Haute Normandie to monohulls such as all the UK J/39 fleet to the keels on nine of the top ten boats at the 2002 Swan Worlds.

Foil to leeward

So in coming up with the foil to leeward, did Welbourn wake up in a cold sweat one night? “I had the idea way back - probably eight or ten years ago,” he says. “We thought - instead of the canting keel, what happens if you reverse the logic and stick a foil down to leeward? Your first thoughts are - that will do something, but does that work in practice? You can do some quick numbers and come up with the results which show what the lift costs and the stability and the displacement reduction should cancel out and give you a positive benefit. And then you say ‘that would be great for flat water – what happens when you start bouncing it through the water?’”

Almost 10 years ago Welbourn put together a design proposal, together with Nigel Irens and Jo Richards, for Ellen MacArthur’s new Kingfisher Open 60. The proposal contained two boats, one a conventional Open 60, the other featuring a very early incarnation of ‘the wings’. This came to nothing, but was the first time Welbourn began to think about the system in earnest.

Following this, in 2002/3, Welbourn designed the 93ft Bols maxi for skipper Gordon Kay (read more about this here). “I talked to Gordon about the idea at the time, but it was too early, too radical and we needed a boat that just worked,” recalls Welbourn. “It was a corporate boat, something that could go around the world - so water ballast, no canting keel, something that would still do 30 knots offshore. But the idea festered. Gordon got pretty excited about it and that’s when we started doing some serious work on it.” Kay now spearheads Dynamic Stability Systems who market the new foil system.

So Welbourn doesn’t like canting keels? “When you stick the bulb out to windward, that strut that is sticking out there is nothing but drag - it’s doing nothing for you. It is drag - full stop. Plus you have one other added appendage down there to give you side force from somewhere. With one added appendage out to leeward - when you need it - then it is doing two things: It is giving you righting moment and it is giving you displacement reduction - so effectively you get one of those for free.”

Bols’ career was sadly short-lived after the boat became embroiled in a legal quagmire following the Dutch distiller being sold to the French drinks group Remy Cointreau. However this did not stop Kay and Welbourn contemplating their next 100 footer, featuring the retractible wings.





Having carried out work in the computer the next step in the foil’s evolution was to be fitted on to a radio controlled model. Thus on a discreet pond in the West Country five years ago a 1m long wing-assisted model could be spied charging around. “I made up some wings and arranged it so that I could plug them in in various positions fore and aft and change the angle of them,” recalls Welbourn. The foil was fitted to one side of the model to see how it performed on that tack versus the non-foiled side. “You couldn’t get any hard and fast numbers of it but you could see how the effects were working out,” admits Welbourn. In addition to Welbourn and Kay, former Prada/Mascalzone Latino composite/ ABN AMRO One composite engineer Will Brooks was also involved at this stage.

Around this time lateral foils were fitted to Sean Langman’s modified Open 60 Grundig for one Rolex Sydney Hobart race, but these were fixed, tiny and mounted at the bow to prevent pitching. As Welbourn points out this is great in flat water but not in waves when the foils are rarely in contact with the water. The sweet spot fore and aft, he has found, tends to be around the pitch centre, just aft of the keel.

“You could see it working. One of the most noticable features was that when you had the foil at the right spot and at the right angle it made it so much easier to sail,” recounts Welbourn. “But we soon ran into problems where the hull was wrong for what we were doing. So I built another one - a 2m model.”

One trial for this second model included fitting twin wings protruding from either side of the boat, as Welbourn figured this might be an interesting configuration for boats blasting downwind, as if the boat ever began heeling to windward the foil to weather would immediately bring it back upright. “On an Open 60 it would be very nice for the autopilot if you have one out to windward, so that if you do an autopilot off, crash gybe you are going straight on to foil stability and you are not going to be lying flat on your side.”

Once again Welbourn returned from the tests with extremely favourable results. “There were times it just picked up and flew. You ended up with control problems because there wasn’t fine enough control on the rudder. It bounced through big waves and it didn’t seem any slower and the rest of the time it just flew. That was a big confidence boost.”

While again it was hard to glean meaningful numbers from this test, he did learn more about the interaction between the foil and the hull. “The hull shape has to be designed so that the natural trim effects of the hull, combined with the trim effects of where the foil is positioned, all work together. If you get it right you end up with a dynamically stable system - and this is really important: Any displacement by the boat is counteracted, it is never amplified. If it starts heeling one way you’ll get less force from the foil and if it heels the other way you get more force from the change in water angle. Plus pitch is always dampened and roll is always significantly dampened.”

At this stage the focus was still on a potential new 100ft maxi. The hull shape for this had developed considerably from Bols - with less relative overall and water plane beam, finer in the bow, more U-shaped, with a different rocker profile and with a double set of chines. The upper chine Welbourn refers to as “the ledge”, as this can be designed to peel water off the hull back into a favourable part of the wave pattern, something that came out of his work within the GBR Challenge design team.

“Getting chines right is a difficult problem,” says Welbourn. “The way they have done the chines on the 60s and the Volvo boats - I don’t think they have got what they are trying to achieve with them, they are too high. You don’t mind immersing the chine and letting it run in the water at the lower speeds at all, provided the back end runs neatly and cleanly.”




After playing on the pond, the team got serious and in May 2004 moved on to tank testing, initially at Southampton University and then at the high speed tank at GKN on the Isle of Wight with a 4m, one tenth scale model of the 100ft maxi. Welbourn says they used the same foil package as Bols for their baseline. “Straightaway we made significant gains on the hull from where Bols was. It was the same in the light stuff - it was 100ft, so it had a bit more wetted surface - but the moment we were powered up it was going 3-4 knots quicker.” And then they added the foil, including some impressive runs at 40 and 50 knots...

Read part two of this article tomorrow